Epitaxial crystal of a high electron mobility transistor (hereinafter, abbreviated to a HEMT) structure has hitherto been composed of a group III-V compound semiconductor thin film formed by metal organic chemical vapor deposition (MOCVD) or molecular beam epitaxy (MBE). For example, a structure has been proposed, in which semiconductor layers including an InAlAs layer and an InGaAs layer are stacked on an InP substrate and an InP layer is further formed as an etch stopper layer. FIG. 6 shows a basic configuration of this HEMT structure including the etch stopper layer composed of the InP layer.
The HEMT structure shown in FIG. 6 includes an undoped InAlAs layer (buffer layer) 102, an undoped InGaAs layer (channel layer) 103, an undoped InAlAs layer (spacer layer) 104, an n-type impurity doped InAlAs layer (electron supply layer) 105, an undoped InAlAs layer (spacer layer) 106, an undoped InP layer (etch stopper layer) 107, an n-type impurity doped InAlAs layer (resistance reducing layer) 108, and an n-type impurity doped InGaAs layer (resistance reducing layer) 109, which are stacked on a semi-insulating substrate 101 made of InP.
The n-type InAlAs layer 108 and n-type InGaAs layer 109 are individually divided by etching. The InP layer 107 is exposed in the etched area. On individual divided parts of the InGaAs layer 109, ohmic electrodes 110 and 111, each of which serves as a source or drain electrode, are formed. On the exposed part of the InP layer 107, a Schottky electrode, which serves as a gate electrode, is formed.
In the above described HEMT structure, etching rate (phosphoric acid or citric acid type etchant) of the InP layer 107 is several tenths or several hundredths of etching rate of the InAlAs layer 108 or InGaAs layer 109. Accordingly, the InP layer 107 is less eroded by the etchant and can provide very high selectivity. The InP layer 107 therefore plays a role in preventing that etching proceeds to the InAlAs layers 105 and 106 under the InP layer 107 to degrade device characteristics of the HEMT such as high frequency characteristics.
In the above described HEMT structure, heterojunction interfaces (hereinafter referred to as heterointerfaces) between materials including different group V elements are formed between the InAlAs layers 106 and 108 and the InP layer 107. In such a device utilizing heterojunction, device characteristics are greatly affected by steepness and flatness of an atomic composition distribution in the heterointerface. For example, in forming this heterointerface, when As is mixed into the InP layer 107 or a transition layer with As and P mixed is formed in the interface because of bad forming conditions, the selectivity of the InP layer 107 as the etch stopper layer is significantly reduced in some cases.
Moreover, when the InP layer 107 is used as the etch stopper layer, it is better that the InP layer 107 is made thinner, and the thickness thereof is usually configured to be about 3 to 6 nm. When the InP layer 107 is formed to be very thin like this, forming conditions of particularly the heterointerface greatly affects the selectivity of the InP layer 107, and optimization of the forming conditions of the heterointerface is therefore important.
As for the formation of the HEMT by MOCVD, supplies of raw material gases for growing the InP layer 107 are adjusted so that density of etched portions of the InP layer 107 is equal to or less than a predetermined value during etching of the InAlAs layer 107, InGaAs layer 108, and the like (for example, JP Hei11-266009A).
In the formation of the HEMT by MBE, irradiation of molecular beams of raw material elements is controlled according to formation of each semiconductor layer by turning on and off shutters and valves provided for respective molecular beam sources. FIG. 7 is a timing diagram showing a procedure of supplying raw materials when a heterointerface between the InAlAs layer and the InP layer is formed by a conventional epitaxial growth method. In step A, molecular beams of In, Al, and As are irradiated to form the InAlAs layer, and in step B, molecular beams of In and P are irradiated to form the InP layer. The In molecular beam is irradiated without being stopped, and irradiation of the As molecular beam and irradiation of the P molecular beams are simultaneously switched to each other. The InAlAs layer and InP layer are thus continuously formed.
At this time, it is assumed that the supplied molecular beams is instantaneously switched since the MBE does not include gas flow unlike the MOCVD. In fact, in terms of a molecular beam of a group III element (for example, Al molecular beam), it has been confirmed that intensity of the molecular beam supplied to the substrate in the step B is not more than 1% corresponding to driving time (usually not more than 1 second) of the shutter.
On the other hand, it has been found that the group V element (As) is easily mixed when the heterointerface is formed because the vapor pressure of the group V element is higher than that of the group III element and molecules thereof remained in a growth chamber even after the supply of the molecular beam is stopped. However, growth conditions (forming conditions of the heterointerface) were determined without any reference to these remaining group V molecules. Accordingly, when opening and closing speed of the valves of the molecular beam sources or the operation states of the shutters changed, the characteristics of the heterointerfaces between the InAlAs and InGaAs layers and the InP layer slightly changed corresponding to the amounts of the remaining group V elements (molecular beam intensities of the remaining group V elements). Moreover, there was a disadvantage that the selectivity was lowered when the InP layer as the etch stopper layer of the HEMT was formed in such a manner.
In order to solve the aforementioned problem, the present invention was made with focusing attention on the amounts of the group V elements remaining at switching of the group V elements in epitaxial growth by MBE. An object of the present invention is to propose an epitaxial growth method achieving formation of a heterointerface with stable characteristics and thus achieving formation of an InP etch stopper layer with high selectivity.